Wall system for catalytic beds of synthesis reactors and relative production process

ABSTRACT

A system ( 8; 9; 50 ) is described of walls for catalytic beds of synthesis reactors ( 1 ), in which there is a wall ( 14 ) in direct contact with a catalytic bed ( 7 ) for containing it, said wall ( 14 ) having a plurality of portions ( 17 ) permeable to the gases and a plurality of portions ( 19; 54; 55 ) impermeable to the gases, said portions ( 17 ) permeable to the gases being equipped with slits ( 18; 52, 53; 60; 70 ) of a size such as to allow the free passage of the synthesis gases through them but not the passage of the catalyst, in which the slits are obtained with milling, water cutting or electro-erosion processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/667,315, filed Dec. 30, 2009, which is a national phase ofPCT/EP2008/005312, filed Jun. 30, 2008, which claims priority toEuropean Patent Application No. 07013091.9, filed Jul. 4, 2007. Theentire contents of these applications is incorporated herein byreference.

FIELD OF APPLICATION

The present invention, in its most general aspect, concerns the field ofheterogeneous catalytic synthesis of chemical compounds through reactorsequipped with fixed catalytic beds crossed by a gaseous flow ofsynthesis gas particularly with radial, axial-radial or axial flow.

In particular the present invention concerns a system of walls for thecontainment of the catalytic bed of a reactor of the type quoted above,and a reactor comprising said system of walls. The invention moreoverregards a process for making said wall system.

PRIOR ART

It is known that reactors with fixed catalytic beds, used forheterogeneous catalytic synthesis of chemical compounds like for exampleammonia and methanol, comprises internal wall systems in particular forthe distribution of the synthesis gases inside the catalytic bed. Suchwall systems are designed and built so as to satisfy certain functionalrequirements necessary for the synthesis reactor to operate correctly,including:

permeability to the gaseous flow of synthesis gases with a suitable headloss, such as to allow optimal distribution thereof on the entirecatalytic bed.

containment and mechanical support of the catalytic mass so as tobalance the thrusts resulting from the mass of the catalyst (due to itsweight and to the differences in thermal expansion between catalyst andcontainment walls) and the thrusts of the gases crossing the catalyticbed.

More specifically, in order to satisfy the aforementioned requirements,it is known to use wall systems for catalytic beds consisting of aplurality of walls each carrying out one or more of the aforementionedfunctions.

FR 2615407, for example, discloses a tubular module wall system for thedistribution of synthesis gas in catalytic beds.

Said system however has many drawbacks, including a certain structuraland assembly difficulty and sensitivity to the phenomenon of nitridingin reactors for the synthesis of ammonia. To remedy the latter problem,it is necessary to use very expensive materials like Inconel® specialsteels (iron-nickel alloys) for low-thickness elements, in order to keepa satisfactory mechanical strength during the operation of the reactor;it is however necessary to make heterogeneous welds (i.e. betweendifferent materials) which are subject to cracks or breaks due tothermal stress caused by the different thermal expansion coefficients ofthe materials used.

SUMMARY OF THE INVENTION

The technical problem underlying the present invention is that ofproviding a system of walls for catalytic beds of synthesis reactorscapable of satisfying the above-listed requirements, in particularpermeability to the synthesis gas and adequate support of the catalyticmass, as well as providing a structural process suitable for making saidwall systems.

This problem is solved by a system of walls for catalytic beds ofchemical reactors characterised in that it comprises a wall ofpredetermined thickness in direct contact with a catalytic bed forcontaining it, said wall having a plurality of gas-permeable portionsand a plurality of gas-impermeable portions, each of said gas-permeableportions being equipped with a plurality of slits having a size such asto allow the free passage of the synthesis gases but avoid the passageof the catalyst, wherein said slits are obtained with a machining methodchosen between milling, water cutting, electro-erosion.

Said wall substantially acts as a containment body of the catalyst, andat the same time permits the flow of gas through said permeable portionsequipped with slits, so that the gas can come into contact with thecatalyst itself.

Preferably, according to the invention, the containment wall has slitsmore extended, in the longitudinal direction, on a side intended to bein contact with the catalyst, obtained with a milling process wherein amilling disc is used (or a plurality of milling discs), of appropriatediameter, and the slits are made with the milling cutter which does notcross through the wall as far as its diameter. In such a way, the slitsare more ample, in the longitudinal direction of the cut, on the millingcutter entrance side with respect to the milling cutter exit side. Byarranging the wall inside the reactor such that the surface with longerslits (i.e. that of milling cutter entrance during the construction)faces the catalyst, a positive effect has been found in terms of processefficiency, since the greater length of the notches at least partlycompensates for the passage section reduction caused by catalyst grains,which can block the notches.

According to another aspect of the invention, the wall has slightlyflared slits obtained with water cutting, the slits thus being narroweron one side of the same wall. It has been found that it is preferable toposition the wall with the narrower slit side towards the catalyst,obtaining the effect of facilitating the expulsion of possible catalystparticles and reducing the possibility of slit obstruction.

The above described processing techniques have shown to be particularlyeffective for making walls with slits for the passage of the gasesaccording to the invention, with some advantages compared to techniques,e.g. laser cutting, which can also be used.

An object of the invention is therefore also a process for making acontainment wall of a catalytic bed, for chemical reactors, comprisingthe step of machining a flat metal sheet, intended to form said wall ora section thereof, by means of a machining technique chosen from amongmilling, water cutting and electro-erosion, obtaining a plurality ofslits having dimensions such to permit the free passage of the gasesthrough them, although avoiding the passage of the catalyst.

According to one aspect of the invention, it is provided to obtain saidslits by milling and with control of the milling cutter (or plurality ofmilling cutters) so that the milling cutter does not cross through thewall as far as its diameter, thus obtaining more extended slits, in thelongitudinal direction of the cut, on the milling cutter entrance sidewith respect to the milling cutter exit side. Preferably, multiple toolsare used, essentially comprising a shaft bearing a certain number ofmilling discs.

The milling processing further has the advantages of low heat generation(less for example than laser cutting); possibility to make slits ofoptimal width as a function of the synthesis process to which thereactor is intended (ammonia, methanol, etc.); good finishing level,which generally does not require further finishing, except for abrushing treatment to remove shavings.

With reference to the water cutting, the invention preferably providesthe use of a multi-head cutting machine, for example with 2, 4, 6 or 8nozzles cutting in parallel. A single head machine, however, can also beemployed.

Water cutting has several advantages: the heat generation is almostnull, thus having no deformation; it does not form any (raze; it allowsto make slits of very small size (even<5 mm); the process is not highlysensitive to possible material non-uniformity; it allows improvedprocess control.

It should be noted that the progressive wear of the water cutting nozzlecan be compensated by acting appropriately on the cutting parameters ofthe machine, and does not involve substantial risks. With traditionalcutting tools, on the other hand, the tool wear cannot be easilycompensated for, and may cause sudden breaking of the tool itself, evenruining (sometimes irreparably) the work piece.

Water cutting, moreover, also permits obtaining with a single processthe piercing, trimming etc. of the edges of the plates intended to formthe aforesaid containment wall.

According to further embodiment of the invention, electro-erosionprocessing can also be effectively employed.

The wall preferably has a modular structure, in particular forfacilitating the insertion by means of manhole in a pre-existingreactor. The above described processing steps are hence equally appliedto wall modules or sections.

Further preferred aspects of the system of walls according to theinvention are now described.

Preferably, the aforesaid containment wall has a thickness in the rangeof 1-10 mm, preferably 3-6 mm. Preferably, the aforementionedcontainment wall also constitutes a mechanical support for saidcatalytic bed through said portions impermeable to the gases. Preferablythe aforementioned containment wall consists of a plurality of modulesfixed together in which each module comprises said portions permeable tothe gases and/or said portions impermeable to the gases.

The slits can be any shape, rectilinear or curvilinear and can bearranged in any way, for example they can have a longitudinal ortransversal extension with reference to the axis of the reactor in anycombination of slits—rectilinear, curvilinear or both.

According to a preferred embodiment of the invention, the system ofwalls according to the invention further comprises a distribution wallequipped with gas-permeable portions arranged in spaced relationshipwith said containment wall so as to create an interspace with it.

Preferably, the gas-permeable portions of the distribution wall consistof a plurality of holes made in said wall. Preferably, saidgas-permeable portions of the distribution wall are arrangedside-by-side towards the portions impermeable to the gases ofcontainment wall so as to avoid the direct impact of the synthesis gasesthat cross the gas-permeable portions with the catalyst. Differently, inthe case of the systems of walls of the aforementioned prior art, it isnot possible to protect the catalyst from the direct impact of thesynthesis gases coming out from the distribution wall.

The aforementioned distribution wall is per se conventional and can beequipped with spacing means, also of the conventional type, to stablymaintain the containment wall of the catalyst and the distribution wallin the desired spaced relationship also in conditions of greatmechanical or thermal stress to which said walls can be subjected insidethe reactor in its normal operating conditions.

The main advantage of the system of walls according to the inventionlies in the simplicity in making it since the gas-permeable portions, inother words the aforementioned slits, are formed directly in thethickness of the modules of the containment wall of the catalysttherefore without having to carry out a plurality of welds as requiredto make the grids in the system of walls of the aforementioned priorart.

Moreover, the presence of portions impermeable to the gases gives theaforementioned containment wall adequate mechanical strength that makesit in general suitable for withstanding the mechanical and thermalstresses in normal operating conditions of the reactor. Advantageously,such mechanical strength can also be suitable for the specificrequirements of use in the reactor for which it is intended by suitablyadjusting the thickness of the containment wall of the catalyst so as tosatisfy such requirements.

For example, the thickness of the containment wall can be such that sucha wall is also able to act as a support of the catalyst through itsportions impermeable to the gases. Alternatively, the catalyst supportfunction can be partially or completely carried out by the distributionwall by suitably adjusting the thickness thereof so as to have asuitable mechanical strength.

It should also be noted that in the system of walls according to theinvention it is not necessary to use expensive materials either for thecontainment wall or for the distribution wall, which means a substantialsaving in production costs.

For example, in the case of use of the system of walls according to theinvention in a reactor for ammonia synthesis, it is possible to use arelatively inexpensive material, like for example stainless steel, tomake both the containment and distribution walls with this obtainingsuitable mechanical strength and resistance to nitriding effects. Inparticular the negative effects on mechanical strength deriving fromsurface nitriding can be compensated by suitably adjusting the thicknessof said walls or, in the case of the containment wall of the catalyst,the distribution and the number of portions permeable to the gases andof portions impermeable to the gases.

Differently, in the case of wall systems of the prior art it isnecessary to use very expensive materials that are highly resistant tonitriding like for example the special steels Inconel® (iron-nickelalloys), for the low-thickness elements (for example the welded rods ofthe grid), in order to keep a satisfactory mechanical strength duringthe operation of the reactor.

It should also be noted that, advantageously, the slits of the portionspermeable to the gases can be arranged according to a predeterminedorder and number along the entire containment wall of the catalyst tosatisfy contingent and specific requirements, without substantiallycompromising the mechanical strength of said wall by doing so.

The present invention also concerns a reactor for the heterogeneoussynthesis of chemical compounds comprising a shell closed at theopposite ends by respective base plates, an opening for the inlet ofsynthesis gases, an opening for the outlet of the reaction products, atleast one catalytic bed and at least one system of walls for said atleast one catalytic bed as described above.

The reactor according to the invention can be of the type with crossingof the gases in the catalytic bed or in the catalytic beds with radial,axial-radial or axial flow.

In particular, in the case of reactors with crossing of said at leastone catalytic bed by the gases with radial or axial-radial flow, thesystem of walls according to the invention can be arranged on an inletside of the gases in said at least one catalytic bed and/or on an outletside of the gases from said at least one catalytic bed with therespective containment walls in contact with the catalyst.

Advantageously, with the system of walls according to the invention, anoptimal distribution of the synthesis gases is obtained, on the gasinlet side, inside the respective catalytic bed thanks to the fact thatthe gases crossing the holes of the distribution wall undergo a loadloss in the interspace between the distribution wall and the containmentwall, which allows the inlet speed of said gases into the catalytic bedto be reduced.

However, it should be noted that other embodiments of the reactor arealso possible in which the distribution wall of the system of wallsaccording to the invention is left out on the gas inlet side in therespective catalytic bed or on the gas outlet side from the respectivecatalytic bed.

In the reactor according to the invention, the systems of walls of thecatalytic beds are supported in a conventional manner inside thereactor. In the case in which a cartridge for containing said catalyticbeds permeable to the gases is envisaged inside the reactor, the systemsof walls for gas inlet according to the invention are arranged at theinner wall of said cartridge with the respective containment walls incontact with the catalyst of the respective catalytic beds.

In the case of reactors with crossing of said at least one catalytic bedby the gases with axial motion, the system of walls according to theinvention can be applied onto the upper gas-inlet base plate and/or ontothe lower gas-outlet base plate of the respective catalytic bed.

Advantageously, on the upper gas-inlet base plate, the system of wallsaccording to the invention can be used in place of the usual protectivegrids of the catalyst at the same time obtaining an optimal distributionof the synthesis gases in the catalytic bed.

Further characteristics and advantages of the present invention shallalso become clear from the following description of some preferredexample embodiments thereof, given for indicating and not limitingpurposes, with reference to the attached figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows a longitudinal section view of a reactor forthe heterogeneous synthesis of chemical compounds incorporating a systemof walls for the inlet of gas into a catalytic bed and a gas outletsystem from said catalytic bed according to the invention,

FIG. 2 schematically shows a cross section view of the reactor of FIG.1,

FIG. 3 schematically shows a perspective view of a section of the systemof walls for the inlet of gas and of the system of walls for the outletof gas of the reactor of FIG. 1,

FIG. 4 schematically shows a perspective view of a section of just thesystem of walls for the outlet of gas of the reactor of FIG. 1,

FIG. 5 schematically shows a perspective view of a section of a systemof walls for the outlet of gas from a catalytic bed according to avariant embodiment of the invention, and

FIG. 6 schematically shows a detail of a system of walls according to afurther embodiment of the present invention.

DETAILED DESCRIPTION

With reference to the aforementioned figures, a reactor for theheterogeneous synthesis of chemical compounds is globally indicated with1.

The reactor 1 is particularly suited for the synthesis of ammonia fromsynthesis gases comprising hydrogen and nitrogen.

The reactor 1 comprises a substantially cylindrical shell 2 closed atthe opposite ends by respective base plates, top 3 and bottom 4. Thereactor 1 is also provided on the top base plate 4 with an opening 5 forthe inlet of a gaseous flow of reactant gases and on the base plate 3with an opening 6 for the outlet of a gaseous flow comprising thereaction products.

Inside the shell 2 an annular catalytic bed 7 is formed of the type withaxial-radial crossing, defined at the side by respective systems ofwalls according to the invention, globally indicated with 8 and 9respectively for the inlet and for the outlet of the gases from thecatalytic bed 7. The relevant characteristics of the systems of walls 8and 9 according to the invention shall be made clearer in the rest ofthe present description.

The catalytic bed 7 is not closed on top to allow it to be crossedaxially by a portion of the flow of reactant gases and it is alsodelimited at the bottom by the bottom base plate 3 of the reactor 1.

In the reactor 1 shown in FIGS. 1 and 2, the system of walls 8 for theinlet of gas is arranged close to the shell 2 whereas the system ofwalls 9 for the outlet of gas is arranged centrally to the reactor 1.Between the shell 2 and the system of walls 8 for the inlet of gas anannular interspace 10 is therefore defined to allow the catalytic bed 7to be crossed radially by a portion of the gaseous flow of reactantgases.

The system of walls 9, on the other hand, is closed on top by a cover 11not permeable to the gases, of known type. A chamber 12 is alsoprovided, extending coaxially to the catalytic bed 7, between the systemof walls 9 and the cover 11, for sending the reaction products comingout from such a catalytic bed to the opening 6 for the outlet of themfrom the reactor 1.

The dashed line 13 represented in FIG. 1 at the top end of the system ofwalls 8 for the inlet of gas, delimits the maximum level that can bereached by the catalyst inside the catalytic bed 7, and defines,together with the systems of walls 8 and 9 and with the bottom baseplate 3, the reaction volume available in the reactor 1.

In FIGS. 1 and 2 the arrows indicate the various paths followed by thegases inside the reactor and in particular through the catalytic bed 7.

In accordance with the present invention, the system of walls 8 for thegas inlet as well as the system of walls 9 for the gas outlet, consistsof two substantially cylindrical walls 14, 15, coaxial and spaced apartso as to form an annular interspace 16 between them.

More specifically (FIGS. 3 and 4), the wall 14 of each of the systems ofwalls 8 and 9 is in direct contact with the catalyst of the catalyticbed 7 for the lateral containment thereof and has a plurality of areasor portions 17 in the form of circumferential bands equipped with aplurality of axial slits 18 (i.e. extending parallel to the longitudinalaxis of the shell 2) alternating with “full” areas or portions 19, inother words without slits, also in the form of circumferential bands.The slits 18 are of a size such as to allow the free passage throughthem of the gases but not of the catalyst of the catalytic bed 7.Regarding this, the slits 18 can have a width of between 0.3 and 2.5 mm,preferably between 0.5 and 1.1 mm.

The slits 18, according to the invention, are obtained with a machiningtechnique chosen from among milling, water cutting and electro-erosion.

For example, the slits are obtained with the chosen technique on themetal sheet intended to form the wall 14 (or a section thereof); saidsheet is then subjected to possible finishing processes and then rolledto form the cylindrical wall 14 or part thereof.

The containment wall 14 is advantageously formed with a suitablethickness and material such as to have good mechanical strength duringthe operation of the reactor also under the effect of nitriding or othercorrosion. As an example, in the case of a reactor for ammoniasynthesis, the containment wall 14 can be formed with a thickness ofbetween 1 and 10 mm, preferably between 3 and 6 mm and the material ofsaid wall can be any material having ordinary resistance to nitridinglike for example stainless steel. Regarding this, it should be notedthat a wall made in this way, although subject to the surface nitridingeffects typical of ammonia synthesis, said effects weakening it overtime, maintains adequate mechanical strength during normal conditions ofuse of the reactor 1. It should also be noted that the containment wall14 can be formed with a suitable thickness so as to act as well as forcontainment also as a mechanical support for the catalytic bed 7 throughthe respective “full” portions 19, i.e. without slits 8.

The distribution wall 15 of each of the wall systems 8 and 9, on theother hand, consists of a metal sheet of suitable thickness equippedwith a plurality of holes 21 arranged, in the example shown here, inparallel axial groups in predetermined spaced apart relationship. Morespecifically, the holes 21 preferably face towards the areas or portionsimpermeable to the gases of the containment wall 14 so as to avoid thedirect impact of the gases with the catalyst. In the example shown inthe figures, such areas or portions impermeable to the gases consist offull areas or portions 19 between the slits 18 of the wall 14.

In the case of the system of walls 8 for the inlet of gas, the functionof the wall 15 is essentially to promote the uniform distribution of thegases going into the catalytic bed 7 as shell be described more clearlyhereafter. The wall 15 is made from a conventional material, for examplestainless steel, and obtained through conventional processes with asuitable thickness according to requirements. Preferably, thedistribution wall 15 is obtained with a suitable thickness to act as amechanical support for the catalytic bed 7. The distribution wall 15 canalso be equipped with spacers (not shown) to stably maintain the spacedrelationship with the containment wall 14 under the effect of mechanicalor thermal stresses during the operation of the reactor 1.

Preferably, each of the aforementioned walls 14 and 15 of a system ofwalls 8 or 9 is formed from longitudinal modules (not shown) of asuitable size to pass through a “manhole” (also not shown) of thereactor 1, said modules then being fixed together (for example welded orbolted) to form the corresponding walls.

As mentioned previously, the reactor 1 comprises a system of walls 8 forthe inlet of the gases into the catalytic bed 7 and a system of walls 9for the outlet of the gases from said catalytic bed, the walls 14 and 15of which have been described up to now with regard to their structure.It should now be noted that, in the case of the system of walls 8 forthe inlet of gas, the distribution wall 15 is arranged outside (goingfrom the axis of the reactor towards the shell) with respect to thecontainment wall 14 of said system 8 of walls and forms with the shell 2the interspace 10. In this way, the gases that cross the annularinterspace 10 pass through the holes 21 of the distribution wall 15 andexpand in the interspace 16 between the walls 14 and 15, thus obtaininga load loss that allows the same gases to be distributed uniformly inthe catalytic bed 7 after having crossed the slits 18 of the wall 14 incontact with the catalyst.

On the other hand, in the system of walls 9 for the outlet of gas, thewall 15 is further in with respect to the containment wall 14.Therefore, the gases that radially cross said catalytic bed 7 come outfrom it by crossing the slits 18 of the wall 14 of the system 9 of wallsand then, passing through the interspace 16, the holes 21 of the wall15, to be collected in the chamber 12 and from here conveyed towards theoutlet opening 6 of the reactor 1.

FIG. 5 shows a system of walls for catalytic beds of synthesis reactorsaccording to a variant embodiment of the invention, said system beingglobally indicated with 50. The elements of the system of walls 50 thatare structurally or functionally equivalent to corresponding elements ofthe systems of walls 8 and 9 described previously shall be attributedthe same reference numerals.

The system of walls 50 shown in FIG. 5 (preferably obtained by milling)comprises a containment wall 14 and a substantially cylindrical wall 15,coaxial and spaced apart so as to form an annular interspace 16 betweenthem. In the containment wall 14 it is possible to distinguish aplurality of slits 52 with a shorter arch, a plurality of slits 53 witha longer arch and portions 54 and 55 impermeable to the gases.

The system of walls 50 is suitable in particular for being applied ontothe outlet side of a catalytic bed crossed by the gases with radial oraxial-radial motion since the wall 15 is arranged inside the containmentwall 14. Of course, by inverting the arrangement of the walls 14 and 15with respect to each other, the system of walls 50 described above canalso be applied to the gas inlet side into the catalytic bed.

FIG. 6 schematically shows an alternative form and arrangement of slitson the containment walls 14 of wall systems according to the invention.In particular, FIG, 6 shows a plurality of slits 60 arranged axially andeach being substantially coil-shaped. Said coil-shaped form isobtainable with water cutting or electro-erosion. Such a coil-shapedarrangement of slits advantageously offers a larger surface availablefor the crossing of the gases for the same mechanical strength of saidcontainment wall 14.

Advantageously, the containment wall 14 has slightly flared and/orlonger slits on one side as a result of the milling or water cuttingprocessing procedure. In this case, the wall 14 is arranged with thesurface having the longer and/or flared slits in direct contact with thecatalytic bed 7. This further expedient reduces the obstruction of theslits themselves, facilitating the expulsion of possible catalyst grainsor parts.

Of course, a man skilled in the art can bring numerous modifications andvariants to the system of walls and to the reactor described above, allof which are covered by the scope of protection of the following claims.

1. A process for making a containment wall of a catalytic beds for achemical reactor, wherein the containment wall is intended for directcontact with the catalytic bed, said wall having a plurality ofgas-permeable portions, and a plurality of gas-impermeable portions, theprocess comprising the steps of: machining a flat metal sheet intendedto form said wall or a section thereof, and cutting a plurality of slitsin said sheet to form said gas-permeable portions, wherein said slitshave a size such as to allow free passage of synthesis gases but avoidthe free passage of catalyst through them.
 2. The process according toclaim 1, wherein the plurality of slits include slits which have agreater length in a longitudinal direction, on a side of saidcontainment wall intended for direct contact with the catalyst withrespect to a length of said slits on an opposite side of saidcontainment wall.
 3. The process according to claim 2, wherein theplurality of slits are cut using a milling cutter, and wherein the sideof the containment wall in which the slits have greater lengthcorresponds to the milling cutter entrance side while the opposite sideof the containment wall corresponds to the milling cutter exit side. 4.The process according to claim 1, wherein the plurality of slits are cutusing a water cutting processing to obtain flared slits.